1. Field of the Invention
The present invention relates to a Co—Cr—Mo-based (=cobalt-chromium-molybdenum-based) alloys and to a production method therefor. In particular, the present invention relates to a production technique for a Co—Cr—Mo-based alloy which can be used for medical implantation devices, for example, prosthetic materials for artificial aggregates, prosthetic implantation parts, artificial hip joints, and materials for joining and fixing bones. The present invention relates to a production technique for a Co—Cr—Mo-based alloy which is superior in compatibility with a living body, corrosion resistance, and wear resistance, and a low magnetic susceptibility which does not exert harmful effects on MRI (=magnetic resonance image) for medical examination diagnosis.
2. Description of Related Art
In examinations of medical images, due to problems of radiation sickness by X-rays exposure, MRI (=magnetic resonance image) is widely used instead of X-ray image. In a case in which an alloy having a high magnetic susceptibility is embedded in a living body of a patient and the patient is set in an MRI apparatus, an image of the living body is obtained by the MRI apparatus. However, in this case, an image of a periphery of a member of the above alloy is distorted or cannot be obtained due to magnetic flux generated from the alloy. As a result, an examination of the MRI or a diagnosis from the MRI (hereinafter “MRI diagnosis”) is difficult.
In medical implantation devices, a Co—Cr—Mo-based (=cobalt-chromium-molybdenu-based) alloy is used as an alloy which is superior in compatibility with a living body, corrosion resistance, and wear resistance. In the future, corrosion resistance and wear resistance of Co—Cr—Mo-based alloys must be improved. However, for example, members of the ASTM (=American Society for Testing and Materials) standard have a high magnetic susceptibility, and thereby are inconvenient for MRI diagnosis.
In a Co—Cr—Mo-based alloy, corrosion resistance and wear resistance are improved remarkably by increasing the amount of included Mo and homogenizing the structure of the Co—Cr—Mo-based alloy. However, plastic workability greatly decreases due to a Mo-rich phase which is hard and brittle is segregated as a second phase, such that working stress increases, or cracking occurs in the second phase. Recently, a melted alloy including Co, 26 to 30 mass % of Cr, 6 to 12 mass % of Mo, and 0 to 0.3 mass % of C is subjected to quench casting in water-cooling copper die, so that a material is obtained. The material is subjected to hot forging so as to have a second phase which is controlled to be finely dispersed in grains having an average diameter of 50 μm or less. As a result, plastic workability of the material is improved (see Japanese Unexamined Patent Application Publication No. 2002-363675).
However, since the composition of the material includes a composition of the ASTM standard, the material of the above composition is estimated to have as high a magnetic susceptibility as those of conventional techniques, and an MRI of a member of the above material is distorted or cannot be obtained. Besides, the hardness of the above composition is estimated to be the same as those of conventional techniques. Due to this, in a case in which the material is used for implanting devices like a ball joint, required wear resistance on a sliding surface thereof cannot be obtained.